1. Field of the Present Invention
The present invention relates to a mounting structure of a semiconductor optical element, and more particularly to a mounting structure of a semiconductor optical element configured to take out an optical output from an opposite side to a PLC (Planar Lightwave Circuit).
2. Description of Related Art
With the advent of the broadband era, various optical devices are used in the optical transmission system. In the trunk system, the introduction of the WDM (Wavelength Division Multiplexing) transmission system has been increasing.
In recent years, the use of the DWDM apparatus (dense wavelength division multiplexing apparatus), which enables higher speed transmission by multiplexing tens of optical wavelengths, has also been spread. In each WDM transmission system, a light source is required for each optical wavelength, and hence the number of necessary light sources is remarkably increased in correspondence with the higher multiplexing.
Further, in recent years, the ROADM (Reconfigurable Optical Add/Drop Multiplexers) configured to add or drop an arbitrary wavelength at each node has been studied in order to be commercialized.
In the optical access system, various module structures have been proposed in order to realize a lower cost optical module. The structures used in the optical access system are roughly classified into two types.
The one is a structure in which full duplex communication is realized in such a way that a beam splitter is introduced in a module and that different wavelengths are used for the incoming and outgoing transmission. In this structure, the beam splitter in which a wavelength filter is introduced, a semiconductor laser, and a light receiving element are introduced in the module. However, in this structure, because of the complicated module structure and the need for strict alignment accuracy, it is considered to be difficult to satisfy the demand of low cost.
As the other structure, in order to provide an optical module at low cost, there is proposed a structure in which a semiconductor laser and a light receiving element are mounted on a platform with a quartz waveguide formed therein.
FIG. 1 shows an example of the structure of the latter optical module. A branching waveguide is formed by a PLC. An optical fiber 14 is mounted, without being adjusted, on a PLC platform 11 by using a V-groove. By this method, the optical fiber core and the waveguide 11a can be positioned at a micrometer level. Further, a semiconductor laser 12 and the optical fiber 14 can be coupled to the waveguide at a sub-micrometer level, without the need of optical axis alignment, in such a way that a mark pattern formed on the semiconductor laser 12 and a mark pattern formed on the PLC platform 11 are matched with each other by using infrared transmitted light.
In the optical module configured in this way, all the work of optical axis alignment can be automated. This greatly contributes to reducing the production cost of the optical module.
The optical module is configured such that an optical filter 15 is arranged at a returning portion of the waveguide 11a, and that only light of a specific wavelength is returned at the returning portion and light of the other wavelengths is received by a PD 16 supported by a PD carrier 17.
As a device in which the passive alignment is used, there is a wavelength tunable laser as shown in FIG. 2. This device is regarded as a key device of the WDM transmission system and is energetically studied in many research institutes. In the structure shown in FIG. 2, three stages of ring resonators 24 are connected in series on a PLC platform 21, and the waveguide is configured to be turned back by a waveguide mirror 25.
A wavelength tunable operation is realized by performing phase control in such a way that the refractive index is locally changed by energizing a heater formed in the each ring resonator. One of the ring resonators is adjusted to match a predetermined period (for example, a period of ITU grid), so that a highly precise wavelength locking operation is realized. An SOA (Semiconductor Optical Amplifier) 22 with phase control, which is a type of the semiconductor optical element, is mounted on the PLC platform 21 with high precision by the passive alignment.
As advantages of this structure, there are listed highly reliable operation based on a structure with no movable portion, production cost reduction based on a simple structure realized by integrating functions as a wavelength tunable laser into one chip, and the like.
Usually, when a semiconductor optical element is mounted by the passive alignment, Fresnel reflection is caused at a connection position because of a difference in the refractive index between the waveguide and the semiconductor optical element. In order to prevent the reflection, there is used a refractive index matching gel with a refractive index close to the refractive index of the waveguide of the PLC platform. In the example shown in FIG. 2, even when a non-reflection coating is not applied to the waveguide, it is possible to realize a stable operation with little ripple in such a way that the non-reflection coating is not applied to air but is applied to the refractive index matching gel 23 to thereby prevent the reflection.
In Japanese Patent Laid Open Publication No. 2001-272582, there is disclosed an invention in which a structure configured to make the end of an adhesive layer recede to the inside from a light emitting surface is adopted, and in which it is thereby prevented that the end of the adhesive layer is extended to be raised to the emitting surface side and projects in the optical path so as to shield the emitted light.